Urinary tract infection (UTI) is a major clinical problem caused primarily by E. coli. Most uropathogenic E. coli strains express type 1 fimbriae, an important urovirulence factor that mediate E. coli adhesion to specific urothelial receptors. Recent data from the investigators' laboratory and others showed that uroplakins Ia and Ib, two major protein subunits for urothelial plaques that cover >80% of the urothelial surface, are the major urothelial receptors of the type 1-fimbriated E. coli. It has been suggested, however, that the binding of the bacteria to urothelial receptors can be blocked by soluble receptors present in the host urine. One soluble receptor candidate is Tamm-Horsfall protein (THP), the most abundant protein in human urine. Their preliminary data indicate that THP can bind via its high-mannose moiety to type 1-fimbriated E. coli, and block the binding of type 1-fimbriated E. coli to urothelial receptors in vitro. These results strongly suggest a possible defensive role of THP against E. coli infection. The main goal of this study is to evaluate the in vivo function of THP, first focusing on its role in host urinary defense against type 1-fimbriated E. coli infection. Toward this end, they will perform three series of experiments: (1) ablate the THP gene by conventional in vivo knockout in which THP gene will be ablated during embryonic development, as well as by inducible knockout in which THP gene will be ablated in adult mice, and examine the susceptibility of transgenic mice to E. coli-induced UTIs; (2) map and characterize the E. coli-binding, high mannose-type glycosylation site(s) of THP isolated from human and mouse urine, and recombinant THP expressed in cultured epithelial cells; and (3) selectively mutate the high-mannose glycosylation site(s) of THP in transgenic mice and test the effects of the absence of this unique glycosylation of THP on urinary defense. These studies should lead to a better understanding of the physiological function of THP, the molecular pathogenesis of urinary tract infections, and the host defense mechanisms of the urinary tract.